In traditional ultrasound medical imaging, or sonography, a single array of ultrasound transducers (herein sometimes called a transmit-receive array) operates both to transmit and receive ultrasound energy. Ultrasound transducer elements transmit ultrasound waves into an object (e.g., tissue). The transmitted energy is scattered and reflected by the tissue, and the scattered and reflected ultrasound energy is received by the same ultrasound transducer elements. The ultrasound transducer converts received ultrasound energy to electrical signals. The received ultrasound signals are analyzed and interpreted through signal processing, generally providing information on location of structures within the tissue.
In medical ultrasound imaging, ultrasound pulses are used in a manner similar to radar, where a pulse is transmitted, and then echoes are received from reflections and from scatter within tissue. In radar (RAdio Detection And Ranging), a short pulse of an electromagnetic (radiofrequency or microwave) carrier wave is transmitted, and echoes or reflections are detected by a receiver, typically co-located with the transmitter. The range of radar is limited by the received signal energy. Analogously, in ultrasound medical imaging, strong, short electrical pulses transmitted by the ultrasound system drive the transducer at a desired frequency in order to achieve good range resolution. The two-way time of flight of received echoes yields range information, and the strength of the received echoes provides information on acoustical impedance (e.g., when a transmitted pulse encounters a structure within tissue with a different density, and reflects back to the transducer). With knowledge of the direction of the transmitted pulse, an ultrasound image, or sonogram, is created. In ultrasound medical imaging, the maximum transmitted power is limited by the voltage tolerated in the system electronic components, and by the peak intensity permitted by safety considerations pertaining to tissue exposure. As in radar, the range is limited by the received signal versus background noise, which is in turn limited by total pulse energy.
Thermoacoustic imaging, sometimes called photoacoustic or optoacoustic imaging, is a technology used in characterizing and imaging materials based on their electromagnetic and thermal properties, having applications in nondestructive testing, clinical diagnostics, medical imaging, life sciences and microscopy. Thermoacoustic imaging uses short pulses of electromagnetic (EM) energy, i.e., the excitation energy, to rapidly heat features within an object that absorb the EM energy (excitation sites). This rapid heating causes the material (e.g., tissue) to increase in pressure slightly, inducing acoustic pulses that radiate from the excitation site as an ultrasound wave. These acoustic pulses are detected using acoustic receivers, such as an array of ultrasound transducers, located at the material's surface. The resulting measurements are analyzed and interpreted through signal processing using time-of-flight and related algorithms, which reconstruct the distribution of absorbed EM energy, sometimes called thermoacoustic computed tomography (TCAT). The result can be displayed to the user as depth profile plots, or as 2-, 3-, or 4-dimensional images.
There are different requirements for clinical ultrasound transducers operating in transmit-receive mode versus receive-only ultrasound transducers employed in thermoacoustic imaging. Clinical ultrasound transducer arrays are constructed and optimized to operate in both transmit and receive ultrasound modes. These ultrasound transducers require high operating efficiency in transmitting and receiving ultrasound energy, which is not a requirement of receive-only transmitters used in thermoacoustic imaging. Clinical ultrasound transducers typically use a lens to provide an optimal depth of focus, and are designed with an optimized frequency of operation. Traditional ultrasound imaging relies upon narrow band reception for image resolution.
By contrast, in thermoacoustic imaging, it is important for the receive-only transducers to receive and process a wide band of frequencies. Thermoacoustic transducer elements and arrays are designed to operate with a high sensitivity in receive-only mode, whereas receive-only transducers do not have to meet the transmission efficiency requirements of transmit-receive elements and arrays. Thermoacoustic image resolution is determined by frequency of the acoustic signal. This frequency in determined by characteristics of the material being imaged, not by the frequency of the emitted electromagnetic energy (“EM”, or excitation, energy). To be able to discriminate a range of materials properties in thermoacoustic imaging (e.g., small and large size structures; imaging shallow materials and deep materials), wide reception bandwidth is critical. A reception bandwidth on the order of 3-6 MHz is considered a fairly wide range, and higher bandwidths are desirable.
One consideration in image formation in both ultrasound imaging and in thermoacoustic imaging is transducer geometry, e.g., geometry of transducer arrays. Different transducer geometries, such as single focused transducer, linear arrays, and two-dimensional arrays, are capable of different modes of image formation. Depending in part on the transducer geometry, the imaging system may for example image single lines, two-dimensional regions, or three-dimensional volumes. The imaging operation also may employ scanning, or motion of the transducers or transducer arrays, to adapt transducer operation to different modes of imaging.
Traditional clinical ultrasound technology indicates locations of features within a tissue or other material, but provides no functional characteristics. On the other hand, thermoacoustic imaging combines absorption contrasts achieved through interaction of the imaged material with the EM excitation energy, with fine ultrasound resolutions characteristic of acoustic reception, thereby enabling deep penetration in in vivo imaging. Thermoacoustic technology can detect dynamic features, and can measure various functional characteristics of anatomy.